A semiconductor diode laser is a monocrystalline pn junction device. In one form of such a device, the pn junction is in a plane disposed in an active region between two parallel rectangular faces of a monocrystalline semiconductor body. Two mutually parallel reflective faces that are perpendicular to the pn junction form a laser cavity. Lasing action is produced by applying a forward voltage across the pn junction, which causes electrons and holes to be injected across the junction, where they recombine and cause a stimulated emission of radiation. Above a given level of electron injection, called the threshold current, the emitted radiation is collected and amplified in the active region. The amplified radiation exits the active region parallel to the pn junction as a monochromatic beam.
Several factors can reduce the laser's efficiency, and hence its output power. One problem is that some electrons and holes which are injected into the active region do not stimulate emission therein. This occurs, for example, when the electron or hole escapes to outside the active region to adjacent portions of the semiconductor body, where it recombines without contributing to laser emission. Similarly, photons produced in the active region can escape from the active region by radiation in a direction not parallel to the pn unction. In addition, some electrons disappear within the active region without producing the desired emission of radiation, such as when they combine with holes at crystal defects.
Sandwiching the active region between two contiguous layers of monocrystalline semiconductive material having a larger energy band gap and a lower index of refraction than the active region, restricts escape of injected electrons and holes and stimulated photons. Lead-europium selenide-telluride laser devices of this type are known. Because the active layer in these devices is not entirely surrounded by the contiguous layers, i.e., is not buried, laser emission. Similarly, photons produced in the active region can escape from the active region by radiation in a direction not parallel to the pn junction. In addition, some electrons disappear within the active region without producing the desired emission of radiation, such as when they combine with holes at crystal defects.
Sandwiching the active region between two contiguous layers of monocrystalline semiconductive material having a larger energy band gap and a lower index of refraction than the active region, restricts escape of injected electrons and holes and stimulated photons. Lead-europium selenide-telluride laser devices of this type are known. Because the active layer in these devices is not entirely surrounded by the contiguous layers, i.e., is not buried, these devices are known to operate in single mode only for currents restricted to less than twice the threshold current.. In addition, these devices are known to have relatively noisy single mode behavior, relatively short tuning per mode, relatively narrow spaced modes, and to be less stable than desired. Moreover, the known sandwiched devices are produced in a relatively complicated procedure, utilizing (1) a one-step molecular beam epitaxy (MBE) process during which each of the various layers are grown onto the substrate, (2) a photolithographic process to form a mesa with the active layer between the confinement and buffer layers, (3) a native oxide passivation process, and (4) a two step metalization.
Buried heterostructure Pb(1-x)Sn(x)Te/PbTe(1-y)Se(y) lasers are known which are fabricated by a two-step liquid phase epitaxy (LPE) technique which does not allow the creation of buried heterostructure lasers or arrays which incorporate europium, strontium or calcium. The inclusion of these materials is important, however, to obtain stable lasers at certain infrared frequencies.